As you say, we don't know yet, and the fundamental change to a fluid drop-in co-op system from an organised, online party vs offline solo system is enough to suggest it's not worth worrying about yet.
All MH veterans can do really is suggest that MH has a history of a vast amount of content for all skill levels. Typically we used to have the single player campaign that had an end boss and credits roll, then post-game content after that. The online campaign also had its own final boss, credits roll, then post-game content, plus regular events and DLC quests. Considering that it was possible to tackle all the online content solo if you wish, you're looking at hundreds of hours of play. I always end up picking a sensible point to move on, usually at the point where multi-monster G-rank quests are pushing me up against the time limit- such quests are obviously designed with hp pools for multiplayer, but they let single players tackle them because some hunters really are just that good. Not me though. Not me.